Hydrogen diffusion in zirconium cladding alloys with an inner liner as quantified by neutron radiography and nanoindentation
Francesco Fagnoni, Pavel Trtik, Jeffrey M. Wheeler, Robert Zubler, Johannes Bertsch, Liliana I. Duarte
Abstract
Embrittlement caused by hydride precipitates in zirconium claddings constitutes a significant risk factor for the integrity of spent nuclear fuel rods, especially during the handling and transport operations necessary before and after long-term intermediate dry storage. Zircaloy-2 is the most commonly used alloy for cladding tubes in boiling water reactors. It is typically manufactured with an inner liner to better cope with issues related to pellet-cladding interaction. Previous work showed that in clad samples possessing a liner, a significant portion of hydrogen tends to migrate towards the liner region during cool-down. In this work, the hydrogen precipitation and distribution after thermal exposures comparable to service was investigated in different commercial claddings for boiling water reactors. The resulting hydrogen distribution was quantified by means of high-resolution neutron imaging at the SINQ spallation neutron source at Paul Scherrer Institut in Switzerland, and the presence of hydrides was subsequently confirmed by metallography and nanoindentation mapping of selected samples. The results show that when a cooling rate of 30 °C/h is applied, the vast majority of the hydrogen tends to accumulate at the liner/substrate interface when a liner is present. At lower cooling rates, the hydrogen tends to distribute more homogenously into the liner material, and the cladding bulk material is left completely hydrogen-free. Neutron radiography of samples after water quenching from homogenization temperatures between 370 °C and 460 °C revealed a significant amount of hydrogen trapped in the hydrides form at the liner/substrate interface at temperatures up to 400 °C. Undissolved hydrides may act as nucleation seeds for hydrides upon cooling, contributing to the observed hydrogen segregation. A model that takes into account the interfacial properties of the region between the liner and the bulk material is proposed.